A multitude of macromolecular protein interactions must properly occur on chromatin to drive functional aspects of chromosome biology like gene transcription, DNA replication, recombination, repair and sister chromatid segregation. Analyzing how proteins interact in vivo with chromatin to direct these activities remains a significant challenge due to the temporal and dynamic nature of their associations. We have recently developed a technique termed Chromatin Affinity Purification with Mass Spectrometry (ChAP-MS) that provides for the enrichment of a native 1 kb section of a chromosome for site-specific identification of protein interactions and associated histone posttranslational modifications (PTMs). In this grant application, we plan to use this cutting-edge approach to define the histone PTMs and proteins regulating transcription at the arsenic response locus in budding yeast. We hypothesize that ChAP-MS will provide for a comprehensive and unbiased identification of all histone modifications and proteins regulating transcription at the arsenic locus in S. cerevisiae. We will pursue the following Aim to test this hypothesis: Use ChAP-MS to define the histone PTMs and proteins regulating transcription at the arsenic response locus in S. cerevisiae.